1. Field of the Invention
The present invention relates to a bonding apparatus and more particularly to a bonding apparatus which has a positioning mechanism for aligning the position of a bonding tool and the position of a substrate.
2. Prior Art
In addition to wire bonding, face-down bonding is performed as a mounting method for connecting the bonding pads on chips such as LSI device, etc., and the bonding leads of substrates. Face-down bonding is a method in which the back surface side of the chip is held by the bonding tool, the front surface side of the held chip (i.e., the surface on the side on which electrode pads are disposed) and the wiring surface of the substrate are caused to face each other, the positions of the electrode pads on the chip and the positions of the bonding leads of the substrate are aligned, and these pads and leads are connected using thermal energy or ultrasonic energy. Flip-chip bonders and COF (chip on film) bonders, etc., are used as face-down bonding apparatuses.
In the positioning that is performed in face-down bonding, the electrode pads on the surface of the chip and the bonding leads on the surface of the substrate, which face each other, must be observed; accordingly, a chip position measurement camera and a substrate position measurement camera are used. For example, in cases where chips are mounted by lowering the bonding tool from above the substrate, a downward-facing camera which observes the area below from above is used as the substrate position measurement camera, and an upward-facing camera which observes the area above from below is used as the chip position measurement camera.
In one disposition method for the two cameras, the downward-facing camera can be disposed in a position that is higher than the height position of the substrate, and the upward-facing camera can be disposed separately from this in a position that is lower than the height position of the substrate.
In Japanese Patent Application Laid-Open (Kokai) No. 2001-176934, an optical device is disclosed in which a prism type mirror that has two reflective surfaces is used, and the light rays are split into the upward direction and downward direction from the prism type mirror, so that the surface of the chip and the surface of the substrate can be simultaneously observed. In this case this optical device can be disposed between the chip and the substrate. Regardless of the method that is adopted, in the example described above, the optical axis along which the chip is observed is oriented upward, and the optical axis along which the substrate is observed is oriented downward, so that these optical axes are separate optical axes. Accordingly, if there is a shift in these optical axes, this has an effect on positioning, and causes positional deviation in the bonding. For example, shifting of the optical axes may be caused by the effects of temperature elevation, etc., during the bonding work, or by a shift in the settings of the optical system, etc., resulting from changes that occur over time. The effects of such shifts in the optical axes on the positional deviation that occurs during bonding will be described below.
First, the ordinary positioning of the chip and substrate in cases where there is no shift in the optical axes is shown in FIG. 26. In order to simplify the description, only the positioning in the X direction shown in the figures will be considered. The chip 10 is held by the bonding tool 12, and the substrate 14 is held on a carrier 16. The bonding tool 12 and carrier 16 can be moved to arbitrary positions in the X direction by respective driving devices not shown in the drawings. The upper and lower cameras 20 used to observe the chip 10 and substrate 14 are disposed between the chip 10 and substrate 14; the position of the chip 10 can be measured by a first camera 26 which has an upward-oriented optical axis 22, and the position of the substrate 14 can be measured by a second camera 28 which has a downward-oriented optical axis 24. Furthermore, in the measurement of positions or distances between positions, it is necessary to consider the magnifications of the respective cameras. Below, however, unless otherwise noted, the system will be described with the distances between positions measured by the cameras calculated as actual distances, i.e., distances on the objective plane.
FIG. 26 shows a case in which the upward-oriented optical axis 22 and downward-oriented optical axis 24 are accurately aligned.
In this case, the center of the visual field of the first camera 26 and the center of the visual field of the second camera 28 coincide; accordingly, the reference position of the chip 10 and the reference position of the substrate 14 may be respectively aligned with these positions. Edge positions of the electrode pads on the chip 10 and edge positions of the corresponding bonding leads of the substrate 14 can be selected as the reference positions used for bonding. In FIG. 26, the upward-oriented optical axis 22 is assumed to be an axis that expresses the center position of the visual field of the first camera 26, and the downward-oriented optical axis 24 is assumed to be an axis that expresses the center position of the visual field of the second camera 28. It is assumed that when the positions of the chip 10 and substrate 14 were measured in this state, these positions were shifted as indicated by the broken line in FIG. 26. In this case, the carrier 16 is first moved until the reference position on the substrate 14 coincides with the downward-oriented optical axis 24 that expresses the center of the visual field of the second camera 28. Then, the bonding tool 12 is moved until the reference position on the chip 10 coincides with the upward-oriented optical axis 22 that expresses the center of the visual field of the first camera 26. In this way, the chip 10 and substrate 14 are positioned, and if there are no other factors involved, positional deviation does not occur in bonding.
If for some reason the upward-oriented optical axis 22 that expresses the center of the visual field of the first camera 26 and the downward-oriented optical axis 24 that expresses the center of the visual field of the second camera 28 are shifted, then the method shown in FIG. 26 is inadequate. Specifically, even if the reference position of the substrate 14 is caused to coincide with the downward-oriented optical axis 24 expressing the center of the visual field of the second camera 28, and the reference position of the chip 10 is caused to coincide with the upward-oriented optical axis 22 expressing the center of the visual field of the first camera 26, the chip 10 will not be bonded to the desired position on the substrate 14. FIG. 27 is a diagram showing a state in which the reference position of the substrate 14 is caused to coincide with the downward-oriented optical axis 24 expressing the center of the visual field of the second camera 28 in a case in which the optical axis 22 is shifted by +Xc with reference to the optical axis 24. Assuming that the chip 10 is in the position indicated by the broken line, then even if this is caused to coincide with the upward-oriented optical axis 22 expressing the center of the visual field of the second camera 28, the reference position of the chip 10 will be shifted by +Xc with respect to the reference position of the substrate 14.
Accordingly, in order to perform bonding in accurate positions with no positional deviation in bonding, it is necessary to measure by some method the amount of deviation +Xc of the optical axis 22 expressing the center of the visual field of the first camera 26 with reference to the optical axis 24 expressing the center of the visual field of the second camera 28. Then, even if there is a deviation between the optical axes of the two cameras, accurate bonding can be performed by aligning the reference position of the chip 10 with a position that is corrected by an amount −Xc with reference to the center of the visual field of the first camera 26.
In regard to methods used to measure the amount of deviation between the first camera and second camera, a method in which reference marks are respectively disposed on two surfaces or reference surfaces of different heights, alignment is accomplished by adjusting the positional relationship between these two reference marks of different heights beforehand, an this is used as a reference, is disclosed in Japanese Patent Application Publication (Kokoku) No. H6-28272 (corresponding to Japanese Patent Application Laid-Open (Kokai) No. H2-244649). More specifically, an optical device which can simultaneously observe the surface of the chip and the surface of the substrate is inserted between these two reference marks whose positional relationship has been arranged beforehand, and the amount of deviation between the first camera and second camera is measured.
Furthermore, Japanese Patent No. 2,780,000 (corresponding to Japanese Patent Application Laid-Open (Kokai) No. H7-7028) discloses that in a semiconductor positioning device which has a first recognition camera and a second recognition camera whose optical axes are oriented facing each other, a target which is disposed between the height of the first recognition camera and the height of the second recognition camera is used as a reference, and the variation of a reference position in the second recognition camera is detected. More specifically, the first recognition camera and second recognition camera with the target sandwiched in between are moved in relative terms so that these cameras can be positioned on the same axis, and the reference positions of the first recognition camera and second recognition camera are measured and stored in memory with the target as a reference. Next, ordinary bonding work is performed, and the variation in the reference positions of the two recognition cameras can be detected by again performing the same measurements as the preceding reference position measurements, and ascertaining if the measured positions have varied from the reference positions stored in memory.
As the tendency toward increased numbers of pins and increased density in chips and substrates has progressed, there has been a requirement for the disposition of chips on substrates with higher precision in face-down bonding apparatuses. In order to dispose chips on substrates with higher precision, it is necessary to reduce the positional deviation in bonding. Accordingly, it is necessary to detect the deviation between the two cameras with greater accuracy as described above.
Among conventional methods for detecting the deviation between the two cameras, the method of Japanese Patent Application Publication (Kokoku) No. H6-28272 requires two targets; furthermore, the work of adjusting and aligning the positional relationship between these targets is required, so that the construction is complicated, and the work takes time.
In the method of Japanese Patent No. 2,780,000, the focal positions of the recognition cameras must be restricted in order to accomplish accurate detection of the deviation. Specifically, in this method, it is necessary that both the focal position of the first recognition camera and the focal position of the second recognition camera be on the target during target recognition, and in the positioning for bonding, it is necessary that the focal position of the camera that recognizes the chip be on the chip, and that the focal position of the camera that recognizes the substrate be on the substrate. In this method, furthermore, bonding must be performed as follows: after the bonding tool is first disposed at the height of the substrate above the recognition camera and recognized, the bonding tool must be raised by a specified amount and moved over the substrate so that the substrate and chip do not collide at the side-surfaces; then, bonding must be performed by again lowering the bonding tool. Accordingly, there are cases in which the bonding speed is lowered by this vertical movement of the bonding tool.
Even assuming that the amount of deviation +Xc between the optical axis of the first camera and the optical axis of the second camera can be accurately measured and corrected, when the bonding work is performed, positional deviation in the bonding occurs in cases where the bonding tool does not move parallel to the optical axis to the bonding height on the substrate from the positioning height of the bonding tool, so that the chip cannot be accurately disposed on the substrate. Such conditions are shown in FIG. 28.
In FIG. 28, the upward-oriented optical axis 22 and the downward-oriented optical axis 24 are accurately aligned. In this case, if the bonding tool 12 moves parallel to the optical axis 22 from the height on the substrate to the height of the focal position of the first camera 26, it is sufficient to align the reference position of the substrate 14 with the center of the visual field of the second camera 28, and then to align the reference position of the chip 10 and the reference position of the substrate 14 with the center of the visual field of the first camera 26, as illustrated in FIG. 26. Now, let us assume that while the bonding tool 12 moves from the height on the substrate to the height of the focal position of the of the first camera 26, the position of the bonding tool 12 deviates by +Xn. In this case, even if the reference position of the substrate 14 is caused to coincide with the downward-oriented optical axis 24 expressing the center of the visual field of the second camera 28, and the reference position of the chip 10 is further caused to coincide with the upward-oriented optical axis 22 expressing the center of the visual field of the first camera 26, the chip 10 will be bonded not in the desired position on the substrate 14, but rather in a position shifted by −Xn on the substrate 14, so that positional deviation occurs in this bonding.
Accordingly, in order to dispose the chip 10 in an accurate position on the substrate 14, it is necessary to measure by some method the amount of deviation caused by the movement of the bonding tool 12, i.e., the amount +Xn by which the bonding tool is shifted while moving to the height of the focal position of the first camera 26, with the position at the height of the substrate as a reference. If this amount of deviation +Xn is measured, then positional deviation in bonding can be suppressed by aligning the reference position of the chip 10 with a position that is corrected by −Xn with reference to the center of the visual field of the first camera 26, so that the chip 10 can be disposed in an accurate position on the substrate 14.
In the methods of Japanese Patent Application Laid-Open (Kokai) No. 2001-176934 and Japanese Patent Application Publication (Kokoku) No. H6-28272, the amount of deviation arising from the movement of the bonding tool cannot be ascertained. Generally, however, the amount of deviation +Xn can be measured by performing trial bonding. Specifically, even if the amount of deviation +Xn is not known, the chip 10 will be shifted by −Xn on the substrate 14 as described above if bonding is performed with the reference position of the substrate 14 caused to coincide with the downward-oriented optical axis 24 expressing the center of the visual field of the second camera 28, and the reference position of the chip 10 further caused to coincide with the upward-oriented optical axis 22 expressing the center of the visual field of the first camera 26. Accordingly, if the sign of the amount of deviation detected by trial bonding is reversed, then the amount of deviation +Xn caused by the movement of the bonding tool can be determined.
However, what can be measured by trial bonding is the amount of deviation between the external shape on the side of the back surface of the chip and the substrate; the amount of deviation between the electrode pads on the chip and the bonding leads of the substrate cannot be accurately measured. Furthermore, when trial bonding is performed during the bonding work, there is a possibility of the waste of substrates and chips, and the performance of trial bonding off-line results in poor efficiency.
Thus, in the prior art, there are restrictions on the construction of the device used to measure the amount of deviation between the two cameras; furthermore, the amount of deviation caused by the movement of the bonding tool cannot be accurately measured. Accordingly, in the positioning of the chip and substrate in bonding, the more accurate correction of these amounts of deviation is difficult, so that the positional deviation in bonding cannot be further reduced.